The present invention relates generally to an implantable medical device for monitoring physiological signals. More specifically, the present invention is directed at providing a device and method for storing physiological signals during episodes of syncope or cardiac arrhythmias for use in the diagnosis of syncope or intermittent arrhythmias.
Syncope, or fainting, as a result of inadequate cerebral blood flow may be caused by a number of neurocardiogenic or cardiac-related factors. A common form of syncope is vasovagal syncope, a type of neurocardiogenic syncope, that is believed to be precipitated by a reflex of the vagal nerves of the left ventricle that causes hypotension and can be accompanied by a sudden drop in heart rate. Though normally not life-threatening, vasovagal syncope may have serious consequences on a patient""s quality of life and can result in injury and hospitalization. Syncope as a result of underlying cardiac dysfunction is a more serious form since the cardiac-related cause may be a life-threatening cardiac illness or arrhythmia.
In many cases, the diagnosis of syncope is a challenge. Nearly half of all cases of syncope are estimated to remain unexplained. One difficulty in diagnosing the cause of syncope is due to the intermittent nature of syncope. A patient may experience a syncopal event only a few times a year making attempts to monitor the cause of syncope in a physician""s office somewhat xe2x80x9chit-or-miss.xe2x80x9d
Commonly used tests for diagnosing syncope may include a head-up tilt table test, an electrophysiological study, 24 to 48-hour ECG monitoring in the form of a Holter monitor, or external event recorders which may be used to record ECG events over a period of several weeks. The limitation of electrophysiological studies and Holter monitoring is again the fact that syncopal episodes, and the cardiac rhythms associated with syncope, may be intermittent and infrequent and therefore entirely missed by short-term monitoring. Even external ECG event recorders that can monitor for several weeks may not provide a long enough period of data collection to record a syncopal event. Another limitation of external devices is patient compliance in keeping skin electrodes well attached and wearing the external unit at all times.
A tilt-table test is used to induce a syncopal event in a clinical setting. A patient lies supine on a table for a period of time after which the table is tilted upward. Vasovagal syncope is characterized by a sudden drop in heart rate and a decrease in blood pressure which can sometimes be induced by a change in position. Tilt-table tests, however, have been found to produce falsely positive results in many cases. False negative results may also be obtained leaving the diagnosis as unexplained. A tilt-table test unfortunately does not always provide reliable results.
The opportunity to diagnose vasovagal syncope or an arrhythmic cause of syncope exists during an actual, spontaneous syncopal event. At that moment, physiological events leading up to the syncopal event can be recorded and analyzed for a more conclusive diagnosis.
Vasovagal syncope can result from at least three types of vasovagal responses: 1) a cardioinhibitory response producing a sudden drop in heart rate, 2) a vasodepressor response producing a sudden drop in blood pressure, and 3) a mixed response with both cardioinhibitory and vasodepressor responses. Vasovagal syncope usually occurs when a patient is upright and gravitational effects impact venous flow back to the heart, further decreasing flow to the brain, precipitating the syncopal event. Therefore, accurate diagnosis and selection of the most effective treatment for vasovagal syncope would include monitoring of patient blood pressure and patient position as well as heart rate or ECG when syncope occurs, as is done during a tilt-table test.
Vasovagal syncope may be treated by pacing the heart at an elevated rate when bradycardia (slow heart rate) occurs or may be treated by administration of a pharmocologic agent. Accurate diagnosis of the cause of syncope is important in determining the proper treatment and preventing recurrent vasovagal syncope.
It would be desirable, therefore, to provide an implantable medical device capable of monitoring heart rate, blood pressure, and patient position for the purpose of diagnosing vasovagal syncope or intermittent cardiac arrhythmias. An implantable monitor could record these physiological signals when a spontaneous syncopal event occurs and could replace or enhance inconclusive tilt-table testing. Incorporating such monitoring capabilities in a device that is capable of treating vasovagal syncope would allow therapy to be applied immediately in response to detected physiological events that normally precede or occur with syncope.
The present invention addresses these needs by providing an implantable medical device capable of sensing a patient""s ECG, heart rate, blood pressure, and body posture or position, for the purposes of diagnosing vasovagal syncope or intermittent cardiac arrhythmias. In one embodiment, the device functions as a monitor by collecting and storing data for later transmission to an external device.
The monitoring device includes an insulated housing with at least two electrodes for sensing heart rate that are incorporated in the housing surface but are electrically insulated from each other and the housing. Within the monitoring device housing, additional physiological sensors may be included, preferably a blood pressure sensor, which may be an infrared sensor, and a position sensor, which may be a three-dimensional accelerometer.
The monitoring device further includes a programmable control system for controlling the monitoring functions, such as recognizing events that will trigger the storage of sensor data; a data acquisition system for sampling and digitizing ECG and sensor signals; a memory for storing data; and a telemetry circuit for transmitting stored data to an external device. A magnet detection circuit may be included for detecting when a patient-held activation device is held over the implanted device so that data storage may be triggered manually the patient when he or she is feeling symptomatic.
In operation, a number of programmable events, or combination of events, may trigger the storage of sensor data such as: an upper heart rate limit, a lower heart rate limit, an upper blood pressure limit, a lower blood pressure limit, a dramatic change in heart rate, or a dramatic change in blood pressure. Upon a detected trigger event or a patient-activated trigger, heart rate or ECG, blood pressure, and body position or posture data may be stored for a predetermined period of time. Data is then downloaded to an external device and used to diagnose a physiologic condition of vasovagal syncope or ventricular bradycardia, tachycardia, or fibrillation.
In an alternative embodiment, the monitoring methods provided by the present invention for diagnosing vasovagal syncope are incorporated in an implantable cardiac stimulation device. The stimulation device includes, in addition to the components of the monitoring device, a set of leads for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses. When vasovagal syncope or a cardiac arrhythmia is detected by the monitoring methods provided by the present invention, operating parameters controlling the stimulation device may be automatically adjusted in order to deliver stimulation to the heart in a way that prevents or treats syncope or the arrhythmia.
The present invention thus provides a device and method for monitoring physiological data for the diagnosis of syncope based on the monitoring techniques used in tilt-table testing with the advantage of recording diagnostic data when a spontaneous syncopal event occurs. Vasovagal syncope due to a hypotensive response with or without a bradycardia response may be diagnosed. Intermittent cardiac arrhythmias may also be diagnosed. Confirmed syncopal or arrhythmic events may be treated by enabling a therapy to be delivered by another device, or the same device when monitoring methods are incorporated in a cardiac stimulation device.